1. Field of the Invention
The present invention generally relates to variable gain amplifiers, and more specifically to high frequency, variable gain amplifiers with a linear-in-dB gain control.
2. Prior Art
The use of variable gain amplifiers (VGAs) is prevalent, for example, in connection with communications, networking, and other electronic devices. Commonly VGAs may be found in various radio frequency. (RF) applications, including the handling of intermediate frequency (IF) and baseband circuits. Many VGAs are designed to provide a constant output signal amplitude even if the input signal amplitude changes. Such changes include the change in the operating parameters of the circuit.
A form of a current-steering VGA 100 is shown with respect to FIG. 1. It comprises an amplifier core of three active devices, transistor 110, transistor 120, and transistor 130. Transistor 110 and transistor 120 form a current mirror stage. The area of transistor 120 is ‘m’ times the area of transistor 110. Device transistor 110 is biased at a DC current I1 produced by an exponential current source 140, having a control input Vctrl. Exponential current sources are well-known in the art and therefore the discussion herein addresses only the usage of such current sources. The collector of transistor 120 is tied to the supply voltage through the load impedance Zload 150. The collector of transistor 130 is tied to the supply voltage while its base is biased at a constant voltage VBIAS. This sets the DC voltage of the common emitter connection. In one embodiment, the input signal is applied to the common emitter of the three devices; in another embodiment the input signal is supplied through transconductance amplifier 160. The current I2 is the tail DC bias current. The transconductance amplifier Gm adds the amplified input signal Gm*Vin to the tail DC bias current I2. The AC output signal is taken across impedance 150 at the collector of transistor 120. The input AC signal is divided among devices transistor 110, transistor 120, and transistor 130, according to their emitter admittances Yi, which depend on the bias current of each device.
The transistors draw the following currents: transistor 110 draws I1, transistor 120 draws m·I1, and transistor 130 draws I2−(m+1)·I1. Since Yi=Ii/VT and the thermal voltage VT=KT/q, the gain of VGA 100 can be reached by using the following equations:Vout/Vin(dB)=20·log10[Y120/(Y110+Y120+Y130)·Gm·Zload]=20·log10{m·I1/[I1+m·I1+(I2−(m+1)])·Gm·Zload}=20·log10[m·Gm·Zload·(I1/I2)]
While I2 is a fixed current value, i.e., independent of Vctrl, I1 has an exponential function dependency of Vctrl such that I1=I10·10(α/20·Vctrl). Substituting this into the gain equation above results in the VGA gain being:Vout/Vin(dB)=20·log10(m·Gm·Zload·I10/I2)+α·Vctrl 
Hence, there is shown a linear-in-dB VGA having a rate of α dB/V. However, even though prior art shows the linear-in-dB capability, the prior art suffers from at least the fact that the current I2 remains constant and hence, regardless of the decrease in gain of the linear-in-dB VGA 100, the I2 current consumption remains the same. It would therefore be advantageous to provide a linear-in-dB VGA that is capable of adjusting I2 current consumption in accordance with the linear-in-dB VGA gain.